The present invention relates to ink jet printers and, more particularly, to ink jet printers utilizing constant volume pumps for supplying ink and regulating the pressure within the ink supply tank.
A typical ink jet printer comprises a print head or image bar having a fluid reservoir and an orifice plate having a plurality of orifices formed therein through which ink flows from the reservoir to impinge upon a medium such as paper. The print head also includes a stimulator for creating mechanical disturbances within the reservoir to effect an orderly break-up of fluid streams from the orifices into fluid droplets. The droplets are selectively charged by electrodes positioned adjacent to the outlets of the orifices, and charged droplets are deflected by deflection fields into catchers which convey the ink back to a supply tank.
The reservoir of the print head is supplied ink from an ink supply tank through a supply line. A pump, connected to the supply line, conveys ink from the tank under pressure to the reservoir. A return line extends from the print head back to the supply tank and includes an outlet valve which is selectively actuated to close the line, or to open it and allow the vacuum within the supply tank to draw ink from the reservoir.
When this system is started up, the pump requires a certain amount of time to pump fluid to the reservoir to create a pressure within the reservoir sufficient to generate fluid streams intense enough to form ink droplets. Prior to the time this pressure is reached, however, the ink will weep through the orifices and may foul the charge plates as well as the environment of the printer surrounding the print head. Numerous systems have been developed to effect an extremely rapid pressure rise within the print head reservoir, thereby minimizing the amount of ink seepage through the orifices at start-up. For example, in Bok et al. U.S. Pat. No. 4,314,264 and in Huliba U.S. Pat. No. 4,318,114, there is disclosed an ink jet printer having a print head communicating with an ink supply line and an ink return line which in turn are connected to a supply tank. A pump on the ink supply line pumps ink from the supply tank to the print head. A solenoid actuated valve is located on the ink return line immediately downstream of the print head.
The ink supply line is connected to a plurality of ink recirculating lines which convey ink from a location downstream of the ink pump to a location upstream of the pump, so that a portion of the ink leaving the pump is returned to the pump inlet, while the remainder flows to the print head. Each of the recirculating lines includes a restriction which creates a different impedance for that line. A valve system is actuated by solenoids such that a selected one of the recirculating lines may be opened at one time.
In operation, the ink supply pump is actuated, pumping ink to the print head and to a selected one of the recirculating lines. The flow rate of ink through the print head at this stage is relatively low and is insufficient to initiate streams of ink droplets through the orifice plate of the print head. Subsequently, the valve system on the ink recirculating line is actuated to divert ink flow through a recirculating line having a higher impedance, thereby reducing the amount of ink recirculating through the line, and increasing the flow of ink through the print head. At this time, the outlet valve adjacent the print head is closed, thereby creating a sudden pressure impulse which travels upstream to the ink in the print head, producing a rapid flow of ink through the orifice plate of the print head and the establishment of the desired jet drop streams. Once the streams of ink drops have been established, the ink flow is diverted through another ink recirculating line to reduce the flow rate through the print head to the optimal flow rate for ink jet printing.
Another method of rapidly increasing the pressure within the print head is disclosed in the McCann et al. U.S. patent application Ser. No. 340,136, filed Jan. 18, 1982, and commonly assigned. That application discloses an ink jet printing system in which an ink pump pumps ink from a supply tank through a supply line to a print head and through a line to an accumulator which is also connected to the print head supply line. As the pump is operated, ink flows to the print head and to the accumulator, where the ink pressure increases with time. The increase in pressure within the accumulator in turn increases the ink pressure within the supply line and print head to a level appropriate for the production of jet drop streams by the orifice plate. Once this pressure level is reached, an outlet valve on a return line from the print head to the supply tank is closed, thereby creating a pressure wave which initiates the production of jet drop streams.
In order to determine the exact time at which the ink pressure within the print head is sufficient to initiate production of jet drop streams, and hence the appropriate time to close the outlet valve, the ink jet printer includes a pressure transducer within the accumulator which is operatively connected to a computer which actuates the solenoid of the outlet valve. However, in order to effect an accurate detemination of the pressure within the print head, it is necessary to eliminate any item from the section of ink supply line extending between the accumulator and print head, such as a filter, which may create a pressure gradient in the line. Accordingly, any contaminants which may enter the ink supply system in the accumulator would travel through the ink supply line to the print head, and perhaps increase the frequency of orifice clogging.
Another method of controlling the ink pressure within a print head is to regulate the pressure within an ink supply tank which is connected to the outlet of a print head by an ink return line. By reducing the gas pressure within the supply tank, a partial vaccum is created within the ink return line, thereby increasing the rate of ink flow from the print head to the supply tank and reducing the amount of ink seepage through the orifice plate.
For example, in the Yu U.S. Pat. No. 4,240,082, there is disclosed a shut down system for an ink jet recorder in which a print head is connected to a supply of ink by an ink supply line having an inlet solenoid, and is connected to a source of vacuum by an outlet line having an outlet valve. A pressure sensing transducer is positioned within the print head to monitor the ink pressure therein. To effect shut down, the inlet valve is closed, thereby stopping the flow of ink to the print head and permitting the fluid pressure therein to drop to a point where the ink jet streams are on the verge of deviating from a desired linear path. At this point, the outlet valve is opened, thereby connecting the print head to a vacuum source which sucks the ink from the head and rapidly reduces the pressure within the head below that at which ink leaves the orifices. Thus, the amount of ink seepage during shut down is minimized.
Accordingly, there is a need for an ink jet printer of a simple yet reliable design, capable of effecting a rapid pressure increase within the print head during start up periods, and a rapid draining of ink from the print head during shut down, thereby minimizing the seepage and splattering of ink during these periods. Furthermore, such an ink jet system should be constructed to provide filtered ink to the print head in order to minimize contamination and clogging of the orifice plate.